Fungal infections are a growing medical threat, particularly for immunocompromised individuals. A molecular understanding of how fungal pathogens sense and respond to their environment will strengthen our ability to identify the Achilles' heel of these organisms for therapeutic purposes. The long-term goal of this research project is to determine how environmental signals regulate morphology and virulence in the fungal pathogen Histoplasma capsulatum. H. capsulatum grows in a filamentous mold (mycelial) form in the soil and a budding yeast form in the host. The transition from the mycelial form to the yeast form is thought to be essential for virulence. Although temperature is known to be a key signal that triggers the conversion between the two forms of H. capsulatum, little is known about the molecular mechanism of how temperature regulates morphology. We hypothesize that H. capsulatum uses conserved signal transduction pathways converging on master transcription factors to regulate morphology in response to temperature. The objective of this proposal is to use genetics and functional genomics to identify key molecules that are required for this process. Our specific aims are (1) to perform a genetic screen using a high throughput colony morphology assay to identify genes required for normal morphology in response to temperature; (2) to use gene expression profiling as a sensitive assay to characterize the function of regulators of morphology; and (3) to further characterize the function of putative regulators of morphology using molecular genetic experiments such as gene inactivation, ectopic expression, and subcellular localization. These studies will generate an understanding of the regulatory circuits used by H. capsulatum to shift its growth program in response to environmental signals. Moreover, the proposed research will elucidate the mechanism of how H. capsulatum regulates virulence properties in response to temperature.